In a hydrometallurgic zinc preparation process zinc-bearing ore is concentrated, roasted and dissolved in sulphuric acid. Besides zinc, also copper, cobalt, nickel and cadmium as well as germanium and antimony are released in the dissolution. These metals or semi-metals, i.e. impurities, are removed from the solution by reduction using zinc powder in a solution purification process. The separation of these metals can be performed by precipitating in one or more phases from a zinc-bearing solution in precipitation reactors or the like. After the aforementioned metals have been removed, the zinc is electrolytically reduced from a zinc sulphate solution. In zinc preparation, the impurities must be removed from a zinc-bearing material to achieve a successful and efficient electrolysis to reduce zinc. Particularly Ge and Sb and the metal ions Co2+ and Ni2+ of the iron group promote the re-dissolving of zinc that stratifies in the electrolysis, resulting in a decrease of the efficiency of electric current.
To improve the separation efficiency of the desired metal and to accelerate the separation in the solution purification process, metallic zinc powder is introduced into the solution, and besides at least one activator. The activator activates the separation of an impurity metal. In addition, the precipitated end product or a property thereof in the precipitation solution can often be used to affect the separation or precipitation rate of metal. The surfaces of the particles of an activator or of a precipitated metal compound have to be purified in order for them to activate the precipitation reaction.
Known in prior art are a number of various ways to optimise metal removal. It is known in prior art to measure the redox potential and the pH values of a metal removal mixture inside a precipitation reactor. By means of the measurement results, variables of the process, such as the consumption of zinc, have been adjusted. However, there is a problem that the measurement electrodes of the redox potential and pH immersed in the reaction mixture get dirty, resulting in an increase in the measurement result errors.
Furthermore, a problem with the continuous metal removal processes has been the adjustment of the zinc powder to be introduced. The adjustment has been difficult, and zinc powder has been introduced in an excess with respect to the reaction.